Strength (including dry strength, wet strength and temporary wet strength, etc.) is a structural property of paper, which mainly depends on the interfiber bond and the nature of fibers in the paper sheet. During the paper making process, the strength properties of paper can be improved by adding enhancers to the paper stock by adjusting the ratio of fibers and pulping to fibrillate as well as by virtue of film-forming properties of a surface sizing agent. An enhancer is usually divided into a dry strength and a wet enhancer.
Di-aldehyde modified cationic, anionic and amphoteric acrylamide polymers, particularly the glyoxalated modified diallyldimethylammonium chloride (DADMAC)/acrylamide copolymers (GPAMs), are widely used as a dry strength and temporary wet strength aid in the paper and paperboard area. Such polymer strength aid is of great interest for paper and paperboard manufacturer since (1) it provides good temporary wet strength together with good dry strength, and (2) it also helps improve the drainage ability and the paper machine runnability. Amphoteric polyacrylamides can impart a good dry strength to paper. In the meantime, such enhancers comprise components of high activity, without any problem in this regard throughout the shelf life. At present, glyoxal modified acrylamide copolymers and amphoteric polyacrylamides are widely-used enhancers.
By comparison with using separately dialdehyde-modified acrylamide polymers such as dialdehyde-modified cationic polyacrylamides (GPAM) (e.g. DADMAC-acrylamide copolymers), and unmodified polyacrylamides (PAM) (e.g., amphoteric polyacrylamides), using the combination of these two types of polymers, e.g., the combination of dialdehyde-modified cationic polyacrylamides with amphoteric polyacrylamides, as the binary enhancers can result in better performances, which combines a plurality of advantages of each component, such as dry strength, wet strength and dehydration capability.
The pH value is important to the property of final enhancing products. If pH of the binary enhancer system can be accurately adjusted to the desired value prior to use, the binary enhancer system can particularly result in the significantly increased ash retention of paper in addition to the improvements in dry strength and wet strength and dehydration capability.
Therefore, there is a need for an on-line control and reaction process for pH adjustment, particularly for a binary enhancer solution, which can flexibly realize the accurate adjustment to pH value and control it within an expected numerical range.